opening overflow reservoir when hot

[The motor guy]

I think I know the answer to this but is it ok to open the overflow reservoir when the engine is still warm. It's just the radiator cap your not supposed to open right? I need to add some coolant and I don't feel like waiting for it to cool down all the way. It's already been shut off for a couple hours.

 

[hhu168]

Warm is OK, hot is not.

 

[HTSS_TR]

For coolant system with both radiator cap and reservoir cap then reservoir cap can be opened anytime.

 

[tinmanSC]

Originally Posted By: HTSS_TR
For coolant system with both radiator cap and reservoir cap then reservoir cap can be opened anytime.

Yup. Open any time. The reservoir is not under pressure.

 

[The motor guy]

That's what I thought. My dad who knows nothing about vehicles made me second guess myself. Thanks guys.

 

[Craig in Canada]

In a lot of systems they are the same thing - this is why the distinction is made about whether the system has both or not. In a lot of cases it's obvious whether you're dealing with a "pressure cap" or just a simple screw top cap of some kind. I wouldn't open any pressure cap when warm/hot if I could help it.

A secondary effect of opening when warm/hot is that you've equalized the pressure at this higher temperature. When the system cools to ambient it will now be under vacuum. In my experience, there can be places which do not leak coolant/steam OUT of the system but they will allow air to be sucked in. This *could* lead to air in your system and cooling problems.

I believe a lot of pressure caps are supposed to be designed to allow air in under vacuum meaning that the location of air ingestion is controlled - low resistance through the cap instead of drawing air in near the block. Again, this may not always be the design, nor may it always be working correctly. I've seen systems with hoses sucked flat when cold.

If at all possible, I would open any part of the system at temps as close to ambient as possible to be on the safe side. There a lot of different designs out there.

 

[CharBaby]

I'm curious about pressurized overflow tanks where there is no radiator cap such as Mazda's system. But, the overflow tank seems to be under pressure(somewhat!)

I personaly don't need to worry about opening these when hot as I don't open anything in the cooling system when it's hot. But others may have concerns.

 

[Craig in Canada]

Originally Posted By: Char Baby
I'm curious about pressurized overflow tanks where there is no radiator cap such as Mazda's system. But, the overflow tank seems to be under pressure(somewhat!)

I personaly don't need to worry about opening these when hot as I don't open anything in the cooling system when it's hot. But others may have concerns.


BMW systems have one cap, and it's on the e-tank (it's an expansion tank, not a "reservoir" or "overflow tank"). It is there to provide an air space that can be pressurized and allow coolant to expand and contract with temperature changes. In my case the cap is a 2bar cap (~30psi) and this tank is under full system heat and pressure. There is some flow through effect, with the e-tank bottom hose connecting straight into the block (not the rad) and a line from the rad coming in the top of the e-tank.

 

[rpn453]

Originally Posted By: Char Baby
I'm curious about pressurized overflow tanks where there is no radiator cap such as Mazda's system. But, the overflow tank seems to be under pressure(somewhat!)


Yeah, the radiator cap is just relocated from the radiator to the reservoir, but functions the same otherwise. The reservoir is completely open to the radiator and at the same pressure.

 

[rshunter]

Originally Posted By: Craig in Canada
Originally Posted By: Char Baby
I'm curious about pressurized overflow tanks where there is no radiator cap such as Mazda's system. But, the overflow tank seems to be under pressure(somewhat!)

I personaly don't need to worry about opening these when hot as I don't open anything in the cooling system when it's hot. But others may have concerns.
BMW systems have one cap, and it's on the e-tank (it's an expansion tank, not a "reservoir" or "overflow tank"). It is there to provide an air space that can be pressurized and allow coolant to expand and contract with temperature changes. In my case the cap is a 2bar cap (~30psi) and this tank is under full system heat and pressure. There is some flow through effect, with the e-tank bottom hose connecting straight into the block (not the rad) and a line from the rad coming in the top of the e-tank.

It's the same set-up on Volkswagens.